Both alternating current and direct current motors have been used as drive sources for hoists together with a variety of means for controlling the speed and direction of rotation of the motors. A common type of motor has been the alternating current wound-rotor induction motor connected to a 60 hertz power supply. Speed and torque of the rotor motor are controlled by switching external resistance into and out of the wound-rotor circuit to thereby vary the speed and torque of the hoist system.
Alternating current adjustable frequency drives for various types of motor applications have come into use more recently. However, due to the exacting load control requirements of hoists, various problems with adjustable frequency drives have prevented their wide spread use in hoist applications. One problem with adjustable frequency drives has been their lack of reliability in producing the necessary torque to control the load at the initiation of hoist operation. A recently developed solution to this problem involves the maintaining of the hoist brake engaged at the beginning of motor operation, applying power from the adjustable frequency drive at a low frequency only sufficient to produce the current level and thereby the torque necessary to control the load on the hoist when the brake is released, and releasing the brake only after the necessary current level is detected. Another problem is that if the speed of the motor deviates from its rated speed relative to the frequency of the applied power, i.e., if the slip angle at which the motor produces maximum torque is exceeded, the motor torque is greatly reduced. Thus, should the motor speed not follow a change in the frequency of the power supply, the motor torque would fall below that required to control a load, resulting in a potentially dangerous situation. As may be appreciated, the loss of ability to control a suspended load presents a particularly hazardous situation in hoist operations which is not incurred in most motor control applications.
Although the improvement relating to the detecting of torque at start-up prior to releasing the brake has made adjustable frequency drive systems far more reliable in their load controlling ability at the initiation of hoist operation, further development of adjustable frequency drive systems to provide them with additional characteristics and reliability is necessary to make them competitive with other types of drive systems. One of these characteristics , as indicated above, is the ability to avoid loss of load control if the motor speed does not follow the frequency or a change in frequency of the power supply.